This invention relates to an improved process for the production of low molecular weight polyalcohols by the catalytic hydrogenation, in two stages of a mixture of different low molecular weight hydroxy aldehydes, hydroxy ketones and polyhydric alcohols of the type formed in the autocondensation of formaldehyde. (This type of mixture will be referred to hereinafter as "formose"). The invention also relates to the use of these polyalcohols for the production of polyurethane plastics.
Since the work of Butlerow and Loew [Ann. 120, 295 (1861) and J. prakt. Chem. 33, 321 (1886)] in the previous century, it has been known that hydroxy aldehydes, hydroxy ketones and polyhydric alcohols are formed in the autocondensation of formaldehyde hydrate (formose synthesis) under the influence of basic compounds such as, for example, calcium or lead hydroxide. Formose has been repeatedly synthesized ever since.
In this connection, reference is made for example of Pfeil, Chem. Berichte 84, 229 (1951), Pfeil and Schroth, Chemische Berichte 85, 303 (1952), R. D. Partridge and A. H. Weiss, Carbohydrate Research 24, 29-44 (1972), Emil Fischer, Formoses of Glycerol Aldehyde and Dioxy Acetone, German Pat. Nos. 822,385; 830,951 and 884,791; U.S. Pat. Nos. 2,121,981; 2,224,910; 2,269,935 and 2,272,378 and British Pat. No. 513,708. These known processes are attended by certain disadvantages (poor volume/time yields, colored secondary products). In recent years, however, new processes have been developed by which it is possible, using conventional catalysts, to obtain high yields of substantially colorless formoses free from troublesome secondary products.
In one of these new processes, the condensation of formaldehyde hydrate is carried out in the presence of soluble or insoluble lead(II)salts or lead(II)ions fixed to high molecular weight substrates, as catalysts and a mixture of hydroxy aldehydes, hydroxy ketones, and polyhydric alcohols of the type formed in the condensation of formaldehyde hydrate, as co-catalyst. In this process, the reaction temperature is generally in the range from 70.degree. to 110.degree. C. and preferably in the range from 80.degree. to 100.degree. C. The pH-value of the reaction solution is adjusted to between pH 6.0 and 8.0 and preferably to between pH 6.5 and 7.0 up to a conversion of from 10 to 60%, preferably from 30 to 50%. Thereafter, the pH is adjusted to between pH 4.0 and 6.0 and preferably to between pH 5.0 and 6.0 by the controlled addition of an inorganic or organic base. By this special control of pH and by subsequent cooling, it was surprisingly possible, despite different residual formaldehyde contents (from 0 to 10% by weight and preferably from 0.5 to 6.0% by weight), to vary reproducibly the product distribution of the corresponding polyol, hydroxy aldehyde and hydroxy ketone mixtures.
After the autocondensation of the formaldehyde hydrate has been interrupted by cooling and/or by deactivation of the lead-containing catalyst with acids, the catalyst and any water present in the products, are removed. For fuller particulars, reference is made to German Offenlegungsschriften Nos. 2,639,084 and 2,732,077.
According to German Offenlegungsschrift No. 2,714,084, another possibility of producing highly concentrated colorless formoses in high volume-time yields is to condense aqueous formalin solutions and/or paraformaldehyde dispersions in the presence of a soluble or insoluble metal catalyst and a co-catalyst produced by the partial oxidation of a dihydric or polyhydric alcohol containing at least two adjacent hydroxyl groups and having a molecular weight of from 62 to 242 or of a mixture of alcohols of this type. The pH-value of the reaction solution is kept between 6.0 and 9.0 up to a conversion of from 5 to 40% by the controlled addition of a base. The pH is subsequently adjusted to between 4.5 and 8.0 until the condensation reaction is terminated, so that it is then lower by 1.0 to 2.0 units than in the first phase of the reaction. The reaction is subsequently terminated by deactivating the catalyst at a residual formaldehyde content of from 0 to 10% by weight and the catalyst is removed.
Formoses of high quality can also be obtained by condensing formaldehyde in the presence of a metal catalyst and more than 10% by weight, based on formaldehyde, of one or more dihydric or polyhydric low molecular weight alcohols and/or relatively high molecular weight polyhydroxyl compounds (cf. German Offenlegungsschrift No. 2,714,104).
According to German Offenlegungsschrift No. 2,721,186, it is particularly economical to use synthesis gases containing formaldehyde directly, i.e. without the necessity of first forming aqueous formalin solutions or paraformaldehyde. In this regard, the synthesis gases, such as are formed in the production of formaldehyde on an industrial scale, are introduced continuously or in batches at temperatures of from 10.degree. to 150.degree. C. into an absorption liquid which consists of water, monohydric or polyhydric low molecular weight alcohols and/or relatively high molecular weight polyhydroxyl compounds and/or compounds capable of enediol formation as co-catalysts and/or soluble or insoluble metal compounds which may be fixed to high molecular weight substrates as catalysts and which have a pH-value of from 3 to 10. The formaldehyde is directly condensed in situ in the absorption liquid (or even in a following reaction tube or a following cascade of stirrer-equipped vessels). The autocondensation of the formaldehyde is terminated at a residual formaldehyde content in the reaction mixture of from 0 to 10% by weight by cooling and/or by deactivating the catayst with acids. Finally, the catalyst is removed.
For numerous applications, mixtures of hydroxy aldehydes, hydroxy ketones and polyalcohols of the type obtained by the processes described above or by conventional processes, have to be converted into mixtures of polyalcohols by reduction of the carbonyl groups (polyol mixtures such as these obtained by the reduction of formose are referred to hereinafter as "formitols"). For example, formose can be directly reduced from aqueous solution with sodium borohydride at temperatures as low as room temperature (cf. R. D. Partridge, A. H. Weis and D. Todd, Carbohydrate Research 24 (1972), 42). Reduction may also be carried out electrochemically.
Processes for the catalytic hydrogenation of sugars and of formose are widely known. The quantities and types of catalysts used differ widely according to the procedure adopted. Thus, L. Orthner and E. Gerisch (Biochem. Zeitung 259, 30 (1933)) described a process for the catalytic hydrogenation of formose, in which a 4% aqueous formose solution is hydrogenated with 170% by weight, based on formose, of Raney nickel in a reaction carried out over a period of 7 to 8 hours at 130.degree. C. under a hydrogen pressure of 120 bars. A process such as this is of course economically unsatisfactory.
U.S. Pat. No. 2,269,935 describes a process in which a solution containing approximately 40% by weight of formose is hydrogenated with 20% by weight of nickel catalyst in a reaction carried out at 120.degree. C. in the acid pH-range under a hydrogen pressure of from 600 to 620 bars. The disadvantage of this procedure lies not only in the high working pressure, but also in the low pH-value which has to be maintained and which leads to products which are colored green by nickel ions.
U.S. Pat. No. 2,224,910 describes a process for the hydrogenation of formose in which a 40% formose solution is hydrogenated with 30% by weight of Raney nickel, based on formose, in a reaction carried out over a period of 4 hours at a pH-value of 7 under a hydrogen pressure of from 140 to 210 bars. This process is also unsatisfactory due to heavy catalyst usage and the long reaction time involved.
Further hydrogenation processes are described in German Pat. Nos. 705,274; 725,842; 830,951; and 1,004,157 and in U.S. Pat. Nos. 2,271,083; 2,272,378; 2,276,192; 2,760,983 and 2,775,621. However, all of these processes are attended by one or more of the following disadvantages: heavy outlay on apparatus and difficult handling due to high hydrogen pressures; heavy outlay on catalyst, based on the hydrogenated product (10 to 200% by weight); and, colored products because of long hydrogenation times (1 to 10 hours).
One feature common to all hitherto known processes is the use of metal or, in some cases, noble metal catalysts. Raney nickel in particular is used, although it only develops its full activity in the alkaline range. However, since formose shows a tendency towards caramellization in alkaline medium and highly discolored products are formed, conventional processes are generally carried out in the weakly acid or neutral pH-range rather than in the alkaline pH-range.
According to the process described in U.S. application Ser. No. 965,645, filed on Dec. 1, 1978, it is possible to hydrogenate formose solutions (optionally in admixture with other natural and/or synthetic sugars) in a strongly alkaline medium in a fast reaction, with minimal outlay on catalyst, under hydrogen pressures of from 100 to 200 bars and at temperatures of from 50.degree. to 250.degree. C. to form colorless solutions of polyol mixtures. In the polyol mixtures thus obtained, the proportion of low molecular weight C.sub.2 -, C.sub.3 -, and C.sub.4 -alcohols is considerably greater than in the formitols obtained by conventional processes. This new process is characterized in that a more than 20% formose solution is pumped in batches into a reactor, which has a temperature of from 100.degree. to 200.degree. C., at such a rate that the concentration of the groups to be reduced does not exceed 2% by weight. Hydrogenation is carried out at a pH of 7.5-12.5 with from 10.sup.-4 to 5.10.sup. -2 % by weight of catalyst (based on the entire formose) and under a hydrogen pressure of from 50 to 300 bars. The catalysts used are in particular metals having atomic numbers of from 23 to 29 (and particularly Raney nickel). However, when the colorless formitol solutions thus produced are concentrated, for example by thin layer distillation in vacuo, they frequently turn yellow in color for reasons as yet unexplained. In this respect, it does not matter how the formose solution was produced and under what conditions it was hydrogenated.